With regard to a casting material for a thin and large die-cast product that is required to have high toughness, such as, for example, an automobile door panel, an Al—Mg alloy having excellent toughness is known. In this case, in order to make crystal grains finer and improve the toughness, the use of an Al—Mg alloy to which at least one of Ti and Zr has been added is known.
When casting a thin and large die-cast product, from the viewpoint of maintaining the flowability of the melt, it is desirable for the pouring temperature (liquidus temperature+superheat temperature) to be high, but in the case of an Al—Mg alloy composition melt, when a high pouring temperature is set, there are the problems that the concentration of Mg in the melt greatly decreases due to oxidation of Mg, etc., and soldering of the melt to a die easily occurs. Because of this, the pouring temperature T is set at, for example, 720° C.≦T≦730° C.
In order to make the crystal grains finer by the addition of Ti and Zr, adding large amounts thereof is effective, but if the amounts are increased with no clear purpose, Ti, etc. becomes saturated at the above-mentioned pouring temperature, thus causing the deposition of crystals such as Al3Ti or Al3Zr.
When increasing the toughness of a die-cast product, there is a limit to the toughness that can be achieved merely by selecting the casting material, and it is impossible to obtain a toughness-improving effect that exceeds this limit.